The Society of Automotive Engineers (SAE) encourages the entire automotive industry to develop a standard data link, preferably a medium-speed (Class B) multiplexed data communications system. The SAE established Recommended Practice J1850 (a set of technical requirements and parameters) has already been accepted by the industry as the communications link.
For J1850, pulse symbols travel over a single-wire bus as shaped pulses that resembles trapezoidal shaped pulses. Each node must have some means for receiving and converting the trapezoidal pulses into square-wave pulses of the kind that can be interpreted by a microcontroller.
The pulse signals travelling over a single wire bus in an automobile environment sometimes encounter uncontrollable ground offset voltages and electromagnetic interferences (EMI) or signal noise. Both ground offset voltages and the signal noise affect trip point locations along rising and falling edges of the trapezoidal shaped pulses. Distortions of the trip points do interject some measure of error in pulse widths during pulse conversion. The information contained in the original trapezoidal shaped pulse changes with respect to pulse width at the trip points of the pulse.
To illustrate the effects of trip-point distortion, consider a variable pulse width modulated (VPWM) bit stream containing trapezoidal shaped pulses. Assume a first data bit represent a J1850 "Short" symbol containing data between the trip points where the pulse width must range between 34 to 96 .mu.s. Assume a second data bit representing a J1850 "long" symbol containing data between the trip points where the pulse width must range between 96-163 .mu.s. It is entirely possible that ground offset voltages and signal noise could cause the original "short" symbol data in the trapezoidal pulse to stretch beyond 96 .mu.s to about 102 .mu.s such that the converted square-wave pulse yields data representing a "long" symbol.
Realizing this problem occurs at real time in an automobile environment and not in a laboratory where environmental interference can be controlled and pulse data can be accumulated using oscilloscope scopes and/or sophisticated mass storage techniques, a search was initiated to find some means of measuring pulse widths of symbol pulses in a manner which would yield symbol data somewhat free of pulse width distortion. That search ended in the measuring system of the present invention.